DESCRIPTION (Applicant's Description Verbatim): Hematopoietic stem cells (HSCs) can be identified on the basis of their ability to extrude fluorescent dyes, a property attributed to expression of transmembrane proteins belonging to the ABC transporter superfamily. It is not known which transporters are expressed on HSCs, or whether ABC transporters have a functional role in stem cell development. We have recently shown that enforced expression of one ABC transporter, the MDR 1 gene product P-glycoprotein (P-gp), resulted in HSC amplification and development of a myeloproliferative disorder in transplanted mice. While these data clearly show that the P-gp transporter can have functional effects in stem cells, expression of P-gp was not necessary for normal stem cell development. In mice with targeted disruptions of MDR 1-like genes, normal HSCs were present and expressed another recently identified ABC transporter named Bcrpl. This conservation of transporter expression in HSCs suggests that transporter function may be required for normal stem cell development. To test this hypothesis, the Bcrp1 gene will be knocked out in murine ES cells, and crosses with mdrl a/b knockout mice will be analyzed for developmental defects in hematopoiesis. The tightly restricted expression pattern of Bcrpl in stem cell populations suggests that it may be a novel marker for stem cell purification. Sorting experiments will be performed to determine whether Bcrpl (BCRP) expression identifies stem cells from hematopoietic and muscle tissue. The last aim of this proposal will define the relationship between deregulated expression of ABC transporters and leukemia by using controllable transgenic expression systems in mice, and through direct study of pediatric leukemia samples. Altogether, it is expected that these experiments will provide novel information regarding the role of ABC transporters in normal and leukemic hematopoiesis.